Multiport memory cell circuit having read buffer for reducing read access time

ABSTRACT

In order to improve ability for driving the potential of a read bit line (192) to a high level, an output terminal (201b) of a memory circuit (21) and a read word line (182) are connected input terminals (204, 205) of a NAND gate (15) respectively. A gate of a transistor (123) is connected to an output terminal (203) of the NAND gate (15) and its source is connected to a power supply line (111) to be supplied with a VDD potential, while its drain is connected to the read bit line (192). MOS transistors (133, 134) are connected in series between the bit line (192) and a grounding conductor (112). Gates of the transistors (133, 134) are connected to the output terminal (203) and the input terminal (205) of the NAND gate (15) respectively. Thus, a time for converting the output terminal from a low level to a high level is reduced, whereby an access time can be reduced.

This is a Division of application Ser. No. 08/626,854, filed on Apr. 2,1996, now U.S. Pat No. 5,684,743, which is a Division of applicationSer. No. 08/383,860, filed on Feb. 6, 1995, U.S. Pat. No. 5,535,159,which is a Division of application Ser. No. 08/105,629, filed Aug. 13,1993, U.S. Pat. No. 5,420,813.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory cell circuit, and moreparticularly, it relates to a multiport memory which has independentlyaccessible write and read ports.

2. Description of the Prior Art

A. Prior Art

(A-1) First Prior Art

FIG. 32 is a circuit diagram showing an exemplary structure of a memorycell circuit 17a of a conventional multiport memory having a singlewrite port and a single read port. A memory circuit 21 is formed by aflip-flop circuit having inverter circuits 14a and 14b whose outputterminals 201a and 201b are connected to input terminals of the invertercircuits 14b and 14a respectively. Thus, data stored in the memorycircuit 21 complementarily appear at the output terminals 201a and 201bof the inverter circuits 14a and 14b.

FIGS. 33 and 34 are diagrams illustrating the structure and theoperation of an inverter circuit 14 shown in FIG. 32 as inverters 14a-c.FIG. 33 is a logic symbol diagram of the inverter circuit 14, and FIG.34 illustrates the inverter circuit 14 which is formed by MOStransistors.

Referring to FIG. 34, gates and drains of a P-channel MOS transistor 51and an N-channel MOS transistor 52 are connected in common respectively.A power supply line 111 is connected to a source of the transistor 51,to supply a VDD potential. On the other hand, a grounding conductor 112is connected to a source of the transistor 52, to supply a GNDpotential. An input terminal 202 of the inverter circuit 14 having sucha structure is connected with the gates of the transistors 51 and 52 incommon, while its output terminal 201 is connected with the drains ofthe transistors 51 and 52 in common.

When low-level data, i.e., the GND potential is supplied to the inputterminal 202 of the inverter circuit 14, the transistor 51 enters aconducting state and the transistor 52 enters a cutoff state, so thatthe output terminal 201 is driven at the VDD potential. When high-leveldata, i.e., the VDD potential is supplied to the input terminal 202, onthe other hand, the transistor 51 enters a cutoff state and thetransistor 52 enters a conducting state, so that the output terminal 201is driven at the GND potential. Namely, the output terminal 201 outputslow-level data which is reverse to the received high-level data.

Referring again to FIG. 32, an input of the inverter circuit 14b goeslow and its output goes high if an output of the inverter circuit 14a isat a low level, for example. Namely, the terminals 201a and 201b of thememory circuit 21 go low and high respectively. Thus, it is possible tohold data in the terminals 201a and 201b of the memory circuit 21, whichis formed by a flip-flop circuit.

The aforementioned memory circuit 21 is provided with write access gatesfor writing data supplied in the write port, to be connected with writebit lines 191a and 191b for transferring write data and a write wordline 181 for selecting a desired memory cell circuit.

The write access gates are formed by N-channel MOS transistors 13a and13b. The transistor 13a has a drain which is connected to the terminal201a of the memory circuit 21, a source which is connected to the writebit line 191a, and a gate which is connected to the write word line 181.Similarly, the transistor 13b has a drain which is connected to theother terminal 201b of the memory circuit 21, a source which isconnected to the write bit line 191b, and a gate which is connected tothe write word line 181.

Further, a read buffer circuit 22a is provided for reading the datastored in the memory circuit 21, and connected with a read bit line 192for transferring data to be read and a read word line 184 for selectinga desired memory cell circuit.

The read buffer circuit 22a is formed by an inverter circuit 14c havingan input terminal 202c and an output terminal 201c, and an N-channel MOStransistor 50 which is a read access gate. The transistor 50 has a drainwhich is connected to the output terminal 201c of the inverter circuit14c, a source which is connected to the read bit line 192, and a gatewhich is connected to the read word line 184. The input terminal 202c ofthe inverter circuit 14c is connected to the terminal 201b of the memorycircuit 21 to connect the read buffer circuit 22a with the memorycircuit 21, thereby forming the memory cell circuit 17a.

The operation of the aforementioned memory cell circuit 17a is nowdescribed. In order to write data, a write driver circuit (not shown)which is connected to the bit lines 191a and 191b is so employed as todrive the bit lines 191a and 191b to low and high levels in response tothe value of the data to be written. At this time, the bit lines 191aand 191b are supplied with logical levels which are complementary toeach other. In other words, the bit line 191b is driven to a high levelwhen the bit line 191a is driven to a low level, and vice versa.

Thereafter the word line 181 is converted to a high level, therebybringing the access gates 13a and 13b into conducting states. Thus, theterminals 201a and 201b of the memory circuit 21 holding the data areelectrically connected with the bit lines 191a and 191b respectively.

Thus, logical levels at the terminals 201a and 201b of the memorycircuit 21 are equalized to those supplied to the bit lines 191a and191b respectively regardless of the logical level of the held data. Thewrite operation is thus completed.

When the word line 181 is converted to a low level after the completionof the write operation, the access gates 13a and 13b are so cut off thatthe memory circuit 21 holds the written data. Even if the values of thebit lines 191a and 191b are thereafter changed, the logical level of theheld data remains unchanged since the bit lines 191a and 191b are notelectrically connected with the memory circuit 21.

Description is now made on a data read operation. The word line 184 isso converted to a high level that the access gate 50 enters a conductingstate. Thus, the output terminal 201c of the inverter circuit 14cforming the read buffer circuit 22a is electrically connected with thebit line 192, so that the logical level which is supplied to the bitline 192 is driven to a logical level which is supplied to the terminal201c, i.e., a logical level complementary to that of the data stored inthe terminal 201b of the memory circuit 21.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that the data read on the bitline 192 is driven to a circuit of a next stage. The read operation iscompleted in the aforementioned manner.

When the word line 184 is converted to a low level after the completionof the read operation, the access gate 50 enters a cutoff state so thatthe bit line 192 is electrically cut off from the read buffer circuit22a.

Thus, the read buffer circuit 22a is so provided in the memory cellcircuit 17a as to prevent the memory data from destruction by the readoperation. Further, it is possible to simultaneously perform read andwrite operations since the data are read and written from independentports.

(A-2) Second Prior Art

FIG. 35 is a circuit diagram showing another conventional memory cellcircuit 17b having a read buffer circuit 22b, which is provided with aCMOS transistor pair in place of the access gate 50 provided in the readbuffer circuit 22a shown in FIG. 32.

The circuit structure of this memory cell circuit 17b is now described.A memory circuit 21 is formed in a similar manner to that shown in FIG.32. The read buffer circuit 22b is formed by an inverter circuit 14c,and read access gates which are defined by a P-channel MOS transistor 54and an N-channel MOS transistor 53.

The transistors 53 and 54 have drains which are connected to an outputterminal 201c of the inverter circuit 14c in common, and sources whichare connected to a read bit line 192 in common. The transistors 53 and54 further have gates which are connected to read word lines 184 and 185respectively.

An input terminal 202c of the inverter circuit 14c is connected to aterminal 201b of the memory circuit 21 to connect the memory circuit 21with the read buffer circuit 22b, thereby forming the memory cellcircuit 17b.

A data write operation of the aforementioned memory cell circuit 17b isidentical to that of the first prior art. As to a data read operation,on the other hand, the read word lines 184 and 185 are in acomplementary relation to each other for transferring non-inversion andinversion signals respectively. The word lines 184 and 185 are convertedto high and low levels respectively, so that the access gates 53 and 54enter conducting states.

Thus, the output terminal 201c of the inverter circuit 14c forming theread buffer circuit 22b is electrically connected to the bit line 192,so that a logical level which is supplied to the bit line 192 is drivento a logical level which is supplied to the terminal 201c, i.e., thatcomplementary to the logical level of data stored in the terminal 201bof the memory circuit 21.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that data read on the bit line192 is driven to a circuit of a next stage. The read operation iscompleted in the aforementioned manner.

After the completion of the read operation, the word lines 184 and 185are converted to low and high levels respectively, whereby the accessgates 53 and 54 enter cutoff states and the bit line 192 is electricallycut off from the read buffer circuit 22b.

(A-3) Third Prior Art

FIG. 36 is a circuit diagram showing a modification of the memory cellcircuit 17b according to the second prior art. The circuit structure ofthis memory cell circuit 17c is now described. A memory circuit 21 hasthe same structure as that of the first prior art. A read buffer circuit22c is formed by P-channel transistors 55 and 56 and N-channel MOStransistors 57 and 58.

A source of the transistor 56 and a drain of the transistor 55 areconnected in common while a source of the transistor 55 is connected toa power supply line 111 to be supplied with a VDD potential. A source ofthe transistor 57 and a drain of the transistor 58 are connected incommon, while the source of the transistor 57 is connected to agrounding conductor 112 to be supplied with a GND potential. Gates ofthe transistors 55 and 58 are connected in common to form the readbuffer circuit 22c.

Gates of the transistors 57 and 56 are connected to word lines 184 and185 respectively. The gates of the transistors 55 and 58 are connectedto the terminal 201b of the memory circuit 21 in common to connect thememory circuit 21 with the read buffer circuit 22c, thereby forming thememory cell circuit 17c.

A data write operation of the memory cell circuit 17c is identical tothat of the first prior art. As to a data read operation, on the otherhand, the read word lines 184 and 185 are in a complementary relation toeach other for transferring non-inversion and inversion signalsrespectively similarly to the second prior art. The word lines 184 and185 are converted to high and low levels respectively, so that thetransistors 56 and 57 enter conducting states.

If a logical level which is supplied to the terminal 201b of the memorycircuit 21 is at a low level, the transistors 55 and 58 enter conductingand cutoff states respectively. Namely, a bit line 192 is driven to theVDD potential, so that high-level data is read.

When the transistors 56 and 57 are in conducting states and the terminal201b of the memory circuit 21 is at a high level, on the other hand, thetransistors 55 and 58 enter cutoff and conducting states respectively.Namely, the bit line 192 is driven to the GND potential, so thatlow-level data is read.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that the data read on the bitline 192 is driven to a circuit of a next stage. The read operation iscompleted in the aforementioned manner.

After the completion of the read operation, the word lines 184 and 185are converted to low and high levels, so that the transistors 56 and 57enter cutoff states and the bit line 192 is electrically cut off fromthe read buffer circuit 22c.

The memory cell circuit 17a according to the first prior art is adaptedto read both high and low level data with a single read bit line 184.However, it is impossible to completely transfer high-level data sincethe read access gate is formed only by the N-channel MOS transistor 50.

When high-level data is read, the potential of the read bit line 192 canbe driven through the N-channel MOS transistor 50 merely to a levelwhich is reduced from the VDD potential by the threshold value of theMOS transistor 50 even if the output of the inverter circuit 14c of theread buffer circuit 22a is at the VDD potential (high level).

Due to such incomplete transfer of the VDD potential, a sufficientoperation margin cannot be ensured as a source voltage supplying the VDDpotential is reduced, leading to a malfunction as the case may be.Namely, the first prior art has such a first problem that a logicallevel which is reverse to that of the data stored in the memory circuit21 may be disadvantageously driven to the circuit of the next stage evenif high-level data is read, since the potential supplied to the bit line192 cannot be completely raised up to the VDD potential.

When high-level data is read on the read bit line 192, further, the readbit line 192 is driven by the P-channel MOS transistor 51 forming theinverter circuit 14c, while a delay at the P-channel MOS transistor 50serving as an access gate is inevitably added to the access time sincethe bit line 192 is driven through the transistor 50. Namely, the firstprior art has such a second problem that the transistor 50 hindersreduction of the access time.

The second prior art is so structured as to solve the first problem.Namely, access gates are defined by transfer gates formed by a CMOStransistor pair employing both P-channel and N-channel MOS transistors54 and 53.

When high-level data is read on the read bit line 192, therefore, theread bit line 192 is completely driven to the VDD potential to cause noproblem as to the operation margin. However, the bit line 192 is driventhrough the access gates similarly to the first prior art, to result inthe second problem of the unnecessary increase of the access time causedby delays in the access gates.

The operation margin is improved also in the third prior art. In thiscase, however, the two transistors are connected in series between theVDD or GND potential and the bit line 192, to lead to a long delay time.

When high-level data is read on the read bit line 192, the read bit line192 is driven to the VDD potential through the P-channel MOS transistors55 and 56. The above problem is particularly remarkable in this casesince the operating speed of a P-channel MOS transistor is generallyslower than that of an N-channel MOS transistor.

Thus, each of the conventional memory cell circuits has the followingproblems in relation to a read operation:

(1) A stable read operation cannot be performed since it is impossibleto ensure a sufficient operation margin.

(2) A delay time in a read buffer circuit for driving a bit line is solarge that an access time is retarded.

SUMMARY OF THE INVENTION

In a first aspect of the present invention, a memory cell circuitcomprises memory means which stores memory logic taking either one ofcomplementary first and second logical values and has a normal outputend for outputting the memory logic, and at least one read means eachincluding an output terminal, a first potential point having a potentialcorresponding to the first logical value, a second potential pointhaving a potential corresponding to the second logical value, a controlterminal for providing a control signal for controlling whether theoutput terminal is supplied with output logic which is identical to thememory logic or brought into a floating state, a first MOS transistorhaving first and second current electrodes which are connected to thefirst potential point and the output terminal respectively, and acontrol electrode, a second MOS transistor, having a first currentelectrode, a second current electrode which is connected to the outputterminal, and a control electrode which is supplied with the controlsignal, and being driven by the control signal, a third MOS transistorhaving a control electrode which is supplied with inverted memory logicwhich is complementary to the memory logic, and first and second currentelectrodes which are connected in series to the second MOS transistorbetween the second potential point and the output terminal, and a logiccircuit which is driven by the control signal for supplying a drivesignal for driving the first MOS transistor to the control electrode ofthe first MOS transistor on the basis of the memory logic.

Preferably, the memory circuit further comprises write means forexternally writing the memory logic in the memory means.

Preferably, the write means comprises a fourth MOS transistor having afirst current electrode which is connected to the normal output end, asecond current electrode which is supplied with prescribed logic to bewritten as the memory logic, and a control electrode which is suppliedwith a write signal for determining whether or not the writing isperformed.

Preferably, the drive signal is a signal which is complementary to thememory logic.

Preferably, the first MOS transistor is of a first conductivity type,and the second and third MOS transistors are of a second conductivitytype which is complementary to the first conductivity type.

Preferably, the first MOS transistor is driven when the second logicalvalue is supplied to the control electrode thereof, and the second andthird MOS transistors are driven when the first logical value issupplied to the control electrodes thereof.

Preferably, the potential corresponding to the first logical value ishigher than the potential corresponding to the second logical value.

Preferably, the logic circuit comprises a logic element for invertingthe logical product of the control signal and the memory logic, whichcorrespond to each other, for obtaining the drive signal.

Preferably, the potential corresponding to the first logical value islower than the potential corresponding to the second logical value.

Preferably, the logic circuit comprises a logic element for taking thelogical sum of the control signal and the memory logic, which correspondto each other, and inverting the same for obtaining the drive signal.

Preferably, the drive signal is supplied to the control electrode of thethird MOS transistor as the inverted memory logic.

Preferably, the memory cell circuit comprises a plurality of read means.

Preferably, the control signal which is supplied in each of theplurality of read means exclusively drives any one of the plurality ofread means.

Preferably, the second current electrode of the second MOS transistor isconnected to the output terminal, the second current electrode of thethird MOS transistor is connected to the first current electrode of thesecond MOS transistor, and the first current electrode of the third MOStransistor is connected to the second potential point.

Preferably, the memory means further has a reverse output end foroutputting a logical value which complementary to the memory logic, andthe reverse output end is connected to the control electrode of thethird MOS transistor.

Preferably, the memory cell circuit further comprises write means forexternally writing the memory logic in the memory means.

Preferably, the write means comprises a fourth MOS transistor having afirst current electrode which is connected to the normal output end, asecond current electrode which is supplied with a prescribed logicalvalue to be written as the memory logic, and a control electrode whichis supplied with a write signal for determining whether or not thewriting is performed.

Preferably, the write means further comprises a fifth MOS transistorhaving a first current electrode which is connected to the reverseoutput end, a second current electrode which is supplied with aprescribed logical value to be written as the memory logic, and acontrol electrode which is supplied with a write signal for determiningwhether or not the writing is performed.

Preferably, the memory means further includes a first inverter having aninput end which is connected to the normal output end and an output endwhich is connected to the reverse output end, and a second inverterwhich is connected in antiparallel with the first inverter.

Preferably, the drive signal is a signal which is complementary to thememory logic.

Preferably, the first MOS transistor is of a first conductivity type,and the second and third MOS transistors are of a second conductivitytype which is complementary to the first conductivity type.

Preferably, the first MOS transistor is driven when the second logicalvalue is supplied to the control electrode thereof, and the second andthird MOS transistors are driven when the first logical value issupplied to the control electrodes thereof.

Preferably, the potential corresponding to the first logical value ishigher than the potential corresponding to the second logical value.

Preferably, the logic circuit comprises a logic element for invertingthe logical product of the control signal and the memory logic, whichcorrespond to each other, for obtaining the drive signal.

Preferably, the potential corresponding to the first logical value islower than the potential corresponding to the second logical value.

Preferably, the logic circuit comprises a logic element for obtainingthe logical sum of the control signal and the memory logic, whichcorrespond to each other, and inverting the same for obtaining the drivesignal.

Preferably, the second current electrode of the second MOS transistor isconnected to the output terminal, the second current electrode of thethird MOS transistor is connected to the first current electrode of thesecond MOS transistor, and the first current electrode of the third MOStransistor is connected to the second potential point.

Preferably, the memory cell circuit comprises a plurality of read means.

Preferably, the control signal which is supplied in each of theplurality of read means exclusively drives any one of the plurality ofread means.

Preferably, the second current electrode of the third MOS transistor isconnected to the output terminal, the second current electrode of thesecond MOS transistor is connected to the first current electrode of thethird MOS transistor, and the first current electrode of the second MOStransistor is connected to the second potential point.

Preferably, the second MOS transistor is also employed in the logiccircuit.

Preferably, the memory cell circuit comprises a plurality of read means.

Preferably, the control signal which is supplied in each of theplurality of read means exclusively drives any one of the plurality ofread means.

Preferably, the logic circuit comprises a second conductivity typefourth MOS transistor having a first current electrode which isconnected to the second current electrode of the second MOS transistor,a second current electrode which is connected to the control electrodeof the first MOS transistor, and a control electrode which is suppliedwith the memory logic, a first conductivity type fifth MOS transistorhaving a first current electrode which is connected to the firstpotential point, a second current electrode which is connected to thefirst current electrode of the fourth MOS transistor, and a controlelectrode which is connected to the control electrode of the second MOStransistor, and a first conductivity type sixth MOS transistor having afirst current electrode which is connected to the first potential point,a second current electrode which is connected to the first currentelectrode of the fourth MOS transistor, and a control electrode which isconnected to the control electrode of the fourth MOS transistor.

In a second aspect of the present invention, a memory cell circuitcomprises a set of memory means each storing memory logic taking eitherone of first and second logical values which are complementary to eachother, and having a normal output end for outputting the memory logic,and at least one read means each including an output terminal, a firstpotential point having a potential corresponding to the first logicalvalue, a second potential point having a potential corresponding to thesecond logical value, a set of control terminals corresponding to thememory means for supplying a set of control signals for controllingwhether the output terminal is supplied with output logic which isidentical to the memory logic or brought into a floating state, at leastone first MOS transistor each having first and second current electrodeswhich are connected to the first potential point and the output terminalrespectively, and a control electrode, a set of second MOS transistorscorresponding to the memory means, each having a first currentelectrode, a second current electrode which is connected to the outputterminal, and a control electrode which is supplied with the controlsignal, and being driven by the control signals corresponding to thememory means respectively, a logic circuit for selecting the memorylogic from a plurality of the memory logics by the set of controlsignals and supplying a drive signal for driving the first MOStransistor to the control electrode of the first MOS transistor on thebasis thereof and, at least one third MOS transistor which is connectedin series between the first current electrodes of the set of second MOStransistors and the second potential point, each having a controlelectrode which is supplied with inverted memory logic which iscomplementary to the one memory logic and first and second currentelectrodes.

Preferably, the memory circuit further comprises a set of write meanscorresponding to the memory means for externally writing the memorylogics in the memory means.

Preferably, each write means comprises a fourth MOS transistor having afirst current electrode which is connected to the normal output end, asecond current electrode which is supplied with a prescribed logicalvalue to be written as the memory logic, and a control electrode whichis supplied with a write signal for determining whether or not thewriting is performed.

Preferably, only one control signal corresponding to the one store meansstoring the one memory logic is activated in the set of control signals.

Preferably, the first MOS transistor is of a first conductivity type,and the second and third MOS transistors are of a second conductivitytype which is complementary to the first conductivity type.

Preferably, the first MOS transistor is driven when the second logicalvalue is supplied to the control electrode thereof, and the second andthird MOS transistors are driven when the first logical value issupplied to the control electrodes thereof.

Preferably, the drive signal is a signal which is complementary to theone memory logic.

Preferably, a potential corresponding to the first logical value ishigher than a potential corresponding to the second logical value.

Preferably, the logic circuit comprises a set of first logic elementscorresponding to the memory means for taking the logical products of thecontrol signals and the memory logics corresponding to each other, and asecond logic element for taking the logical sum of outputs of the set offirst logic elements and inverting the same for obtaining the drivesignal.

Preferably, the potential corresponding to the first logical value islower than the potential corresponding to the second logical value.

Preferably, the logic circuit comprises a set of first logic elementscorresponding to the memory means for taking the logical sums of thecontrol signals and the memory logics corresponding to each other, and asecond logic element for taking the logical product of outputs of theset of first logic elements and inverting the same for obtaining thedrive signal.

Preferably, the drive signal is supplied to the control electrode of thethird MOS transistor as the inverted memory logic.

Preferably, the memory cell circuit comprises a plurality of the readmeans.

Preferably, the set of control signals which are supplied in each of theplurality of read means exclusively drive any one of the plurality ofread means.

Preferably, the first MOS transistor is provided in correspondence tothe set of second MOS transistors.

Preferably, the third MOS transistor is provided in correspondence tothe set of second MOS transistors.

Preferably, the second current electrodes of the set of second MOStransistors are connected to the output terminal in common, the firstcurrent electrodes of the set of second MOS transistors are connected tothe second current electrodes of corresponding third MOS transistorrespectively, and the first current electrodes of the third MOStransistor is connected to the second potential point.

Preferably, the memory cell circuit comprises a single first MOStransistor.

Preferably, the memory cell circuit comprises a single third MOStransistor.

Preferably, the second current electrodes of the set of second MOStransistors are connected to the output terminal in common, the firstcurrent electrodes of the set of second MOS transistors are connected tothe second current electrode of the third MOS transistor in common, andthe first current electrode of the third MOS transistor is connected tothe second potential point.

In the memory cell circuit according the first or second aspect of tothe present invention, the first MOS transistor connects the outputterminal to the first potential point. The third MOS transistor iscontrolled by the inverted memory logic which is complementary to thememory logic, to connect the output terminal to the second potentialpoint. The second MOS transistor is connected in series with the thirdMOS transistor, to be ON-OFF controlled by the control signal.

According to the first or second aspect of the present invention, thepotential of the output terminal fully swings from the second logicalvalue to the first logical value, whereby the logical value can bestably read from the output terminal with a wide operation margin evenif potential difference corresponding to the first and second logicalvalues is reduced. Further, the operating time for changing the logicalvalue of the output terminal from the second logical value to the firstlogical value is reduced, whereby reduction of the time required for theoperation can be expected. Further, the control signal requires noinverted signal, whereby the number of wires can be reduced instructure.

Particularly according to the second aspect of the present invention, itis possible to reduce capacitance being parasitic on the output terminalwhen the output terminal is driven to the potential corresponding to thefirst logical value by a single first MOS transistor, thereby reducingthe access time. Thus, a charge/discharge current flowing in the outputterminal is also reduced, to save the power.

In a third aspect of the present invention, a memory cell circuitcomprises memory means for storing memory logic taking either one offirst and second logical values which are complementary to each otherand having a normal output end for outputting the memory logic, and readmeans including an output terminal, a first potential point having apotential corresponding to the first logical value, a second potentialpoint having a potential corresponding to the second logical value, acontrol terminal for supplying a control signal taking the first logicalvalue for supplying the output terminal with output logic which isidentical to the memory logic while taking the second logical value forbringing the output terminal into a floating state, a first MOStransistor having first and second current electrodes which areconnected to the first potential point and the output terminalrespectively, and a control electrode, a second MOS transistor havingfirst and second current electrodes which are connected to the secondpotential point and the output terminal respectively, and a controlelectrode, and a logic circuit for supplying first and second cutoffsignals for bringing at least one of the first and second MOStransistors into a cutoff state to the control electrodes of the firstand second MOS transistors respectively on the basis of the controlsignal and the memory logic.

Preferably, the memory cell circuit further comprises write means forexternally writing the memory logic in the memory means.

Preferably, the write means comprises a third MOS transistor having afirst current electrode which is connected to the normal output end, asecond current electrode which is supplied with a prescribed logicalvalue to be written as the memory logic, and a control electrode whichis supplied with a write signal for determining whether or not thewriting is performed.

Preferably, the first MOS transistor is of a first conductivity type,and the second MOS transistor is of a second conductivity type which iscomplementary to the first conductivity type.

Preferably, the first MOS transistor enters a cutoff state when thefirst logical value is supplied to the control electrode thereof, andthe second MOS transistor enters a cutoff state when the second logicalvalue is supplied to the control electrode thereof.

Preferably, the first and second cutoff signals are complementary toeach other when the control signal performs control of supplying theoutput terminal with output logic which is the same logic as the memorylogic, and both of the first and second cutoff signals are signals whichare complementary to the memory logic when the control signal performscontrol of bringing the output terminal into a floating state.

Preferably, the logic circuit comprises a first logic element receivingthe control signal and the memory logic and inverting the logical sumthereof for producing the first cutoff signal.

Preferably, the logic circuit further comprises a second logic elementfor receiving the control signal and the first cutoff signal andproducing the cutoff signal by the logical sum thereof.

Preferably, the second logic element is an AND gate.

Preferably, the second logic element comprises a NOR gate having a firstinput end receiving the first control signal, an output end which isconnected to the control electrode of the second MOS transistor, and asecond output end, and an inverter for inverting the control signal andsupplying the same to the second output end of the NOR gate.

Preferably, the memory means has a reverse output end for outputting alogical value which is complementary to the memory logic, and the logiccircuit further comprises a second logic element having a first inputend which is supplied with the control signal and a second input endwhich is connected to the reverse output end of the memory means formaking the second cutoff signal by the logical sum of logical valueswhich are supplied to the first and second input ends thereof.

Preferably, the write means further comprises a fourth MOS transistorhaving a first current electrode which is connected to the reverseoutput end, a second current electrode which is supplied with aprescribed logical value to be written as the memory logic, and acontrol electrode which is supplied with a write signal for determiningwhether or not the writing is performed.

In the memory cell circuit according to the third aspect of the presentinvention, the first and second MOS transistors connect the outputterminals to the first and second potential points respectively. Thefirst and second MOS transistors are turned on/off in accordance withthe first and second cutoff signals respectively, so that at leasteither one is regularly in an OFF state.

According to the third aspect of the present invention, further, thepotential of the output terminal fully swings from the first logicalvalue to the second logical value and vice versa, whereby the logicalvalue can be stably read from the output terminal with a wide operationmargin even if potential difference corresponding to the first andsecond logical values is reduced. Further, both of the operating timesfor changing the logical value of the output terminal from the secondlogical value to the first logical value and vice versa are reduced,whereby reduction of the time required for the operation can beexpected.

Accordingly, an object of the present invention is to provide a memorycell circuit of a multiport memory having a read buffer circuit which isimproved in ability for driving a potential of a read bit line to a VDDpotential, thereby attaining a high-speed access time.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the structure of a first embodimentaccording to the present invention;

FIG. 2 is a logic symbol diagram of a NAND gate;

FIG. 3 is a circuit diagram showing an exemplary structure of a NANDgate;

FIG. 4 is a circuit diagram showing the structure of a modification ofthe first embodiment according to the present invention;

FIG. 5 is a circuit diagram showing the structure of a second embodimentaccording to the present invention;

FIG. 6 is a circuit diagram showing the structure of a modification ofthe second embodiment according to the present invention;

FIG. 7 is a circuit diagram showing the structure of a third embodimentaccording to the present invention;

FIG. 8 is a circuit diagram showing the structure of a modification ofthe third embodiment according to the present invention;

FIG. 9 is a circuit diagram showing the structure of a fourth embodimentaccording to the present invention;

FIG. 10 is a circuit diagram showing the structure of a fifth embodimentaccording to the present invention;

FIG. 11 is a circuit diagram showing the structure of a sixth embodimentaccording to the present invention;

FIG. 12 is a circuit diagram showing the structure of a seventhembodiment according to the present invention;

FIG. 13 is a logic symbol diagram of an AND-NOR composite circuit;

FIG. 14 is a circuit diagram showing an exemplary structure of anAND-NOR composite circuit;

FIG. 15 is a circuit diagram showing the structure of a modification ofthe seventh embodiment according to the present invention;

FIG. 16 is a circuit diagram showing the structure of an eighthembodiment according to the present invention;

FIG. 17 is a circuit diagram showing the structure of a modification ofthe eighth embodiment according to the present invention;

FIG. 18 is a circuit diagram showing the structure of a ninth embodimentaccording to the present invention;

FIG. 19 is a circuit diagram showing the structure of a tenth embodimentaccording to the present invention;

FIG. 20 is a plan view for illustrating an effect of the presentinvention;

FIG. 21 is an enlarged view showing a basic cell stage 3;

FIG. 22 is a circuit diagram showing the basic cell stage 3;

FIG. 23 is a wiring diagram showing the prior art;

FIG. 24 is a wiring diagram showing the first embodiment of the presentinvention;

FIG. 25 is a wiring diagram showing the second embodiment of the presentinvention;

FIG. 26 is a wiring diagram showing the third embodiment of the presentinvention;

FIG. 27 is a wiring diagram showing the seventh embodiment of thepresent invention;

FIG. 28 is a wiring diagram showing the eighth embodiment of the presentinvention;

FIG. 29 is a circuit diagram showing the structure of an eleventhembodiment according to the present invention;

FIG. 30 is a circuit diagram showing the structure of a twelfthembodiment according to the present invention;

FIG. 31 is a circuit diagram showing the structure of a thirteenthembodiment of the present invention;

FIG. 32 is a circuit diagram showing the prior art;

FIG. 33 is a logic symbol diagram of an inverter circuit;

FIG. 34 is a circuit diagram showing an exemplary structure of aninverter circuit;

FIG. 35 is a circuit diagram showing the prior art; and

FIG. 36 is a circuit diagram showing another prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

B. Description of Embodiments for Improving Single Conductivity TypeTransistors

In general, P-channel MOS transistors are slower in operation thanN-channel MOS transistors, and hence improvement is preferably made asto the P-channel MOS transistors. The following first to tenthembodiments are described in relation to application of the presentinvention to P-channel MOS transistors.

As supplementarily explained in relation to some of the embodiments, thepresent invention is not exclusively applied to improvement as toP-channel MOS transistors.

(B-1) First Embodiment

FIG. 1 is a circuit diagram showing the structure of a memory cellcircuit 171 of a multiport memory having a single write port and asingle read port.

This memory cell circuit 171 comprises a memory circuit 21 and a readbuffer circuit 221.

The memory circuit 21 is formed by a flip-flop circuit having invertercircuits 14a and 14b whose output terminals 201a and 201b are connectedto input terminals of the counter inverter circuits 14b and 14arespectively, to store data. The memory circuit 21 is provided withwrite access gates for writing data, being received in the write port,in the memory cell circuit 171, and is connected with write bit lines191a and 191b for transferring the write data and a write word line 181for selecting a desired memory cell circuit.

The write access gates are formed by N-channel MOS transistors 13a and13b. The transistor 13a has a drain which is connected to the terminal201a of the memory circuit 21, and a source which is connected to thewrite bit line 191a. The transistor 13a further has a gate which isconnected to the write word line 181.

Similarly, the transistor 13b has a drain which is connected to theterminal 201b of the memory circuit 21, and a source which is connectedto the write bit line 191b. The transistor 13b further has a gate whichis connected to the write word line 181.

Further, the read buffer circuit 221, which is adapted to read the datastored in the memory circuit 21, is connected with a read bit line 192for transferring the data to be read and a read word line 182 forselecting a desired memory cell circuit.

The read buffer circuit 221 is formed by a NAND gate 15, a P-channel MOStransistor 123, and N-channel MOS transistors 133 and 134.

The transistor 123 has a source which is connected to a power supplyline 111 to be supplied with a VDD potential, while the transistor 133has a source which is connected to a grounding conductor 112 to besupplied with a GND potential. The transistor 134 has a source which isconnected with a drain of the transistor 133. The transistors 123 and134 have drains which are connected to the bit line 192 in common. Thetransistors 123 and 133 have gates which are connected to an outputterminal 203 of the NAND gate 15, while the transistor 134 has a gatewhich is connected to the word line 182. The NAND gate 15 has an inputterminal 204 which is connected with the terminal 201b of the memorycircuit 21, and another input terminal 205 which is connected with thegate of the transistor 134.

FIGS. 2 and 3 illustrate the structure and the operation of the NANDgate 15 shown in FIG. 1. FIG. 2 is a logic symbol diagram of the NANDgate 15, and FIG. 3 illustrates an exemplary structure of the NAND gate15 which is formed by MOS transistors.

Referring to FIG. 3, P-channel MOS transistors 121 and 122 have drainswhich are connected to the output terminal 203 in common. Thetransistors 121 and 122 further have sources which are connected to thepower supply line 111, to be supplied with the VDD potential. AnN-channel MOS transistor 132 has a source which is connected to a drainof an N-channel MOS transistor 131. The transistor 131 has a sourcewhich is connected with the grounding conductor 112 to be supplied withthe GND potential, while the transistor 132 has a drain which isconnected with the output terminal 203.

The transistors 121 and 131 have gates which are connected to the inputterminal 204 in common, while the transistors 122 and 132 have gateswhich are connected to the input terminal 205 in common, to form theNAND gate 15.

The operation of the NAND gate 15 is now described. When the inputterminal 204 of the NAND gate 15 is supplied with low-level data (GNDpotential), the transistor 121 enters a conducting state and thetransistor 131 enters a cutoff state, whereby the output terminal 203 isdriven to the VDD potential regardless of the value at the inputterminal 205, to output high-level data.

When the input terminal 205 is supplied with low-level data (GNDpotential) similarly to the above, the transistor 122 enters aconducting state and the transistor 132 enters a cutoff state, wherebythe output terminal 203 is driven to the VDD potential regardless of thevalue at the input terminal 204, to output high-level data.

When both of the input terminals 204 and 205 are supplied withhigh-level data, the transistors 121 and 122 enter cutoff states and thetransistors 131 and 132 enter conducting states, whereby the outputterminal 203 is driven to the GND potential, to output low-level data.

Namely, the NAND gate 15 so operates that its output goes high when atleast either one of the two input terminals 204 and 205 is supplied withlow-level data while outputting low-level data only when both of theterminals 204 and 205 are supplied with high-level data.

Referring again to FIG. 1, the operation of the memory cell circuit 171is described. A data write operation of this circuit is absolutelyidentical to that of the first prior art. A write driver circuit (notshown) which is connected with the bit lines 191a and 191b is employedto drive the bit lines 191a and 191b to low or high levels in responseto the value of the written data.

The bit lines 191a and 191b are driven to be complementary to eachother. Namely, the bit line 191b is driven to a high level when the bitline 191a is driven to a low level, and vice versa.

Thereafter the word line 181 is converted to a high level, so that theaccess gates 13a and 13b enter conducting states. Thus, the terminal201a of the memory circuit 21 holding data is electrically connectedwith the bit line 191a, while the terminal 201b of the memory circuit 21is electrically connected with the bit line 191b. Therefore, logicallevels which are supplied to the terminals 201a and 201b of the memorycircuit 21 are equalized to those supplied to the bit lines 191a and191b respectively, regardless of the logical levels of the data heldtherein. The data write operation is completed in the aforementionedmanner.

After the completion of the data write operation, the word line 181 isconverted to a low level to cut off the access gates 13a and 13b, sothat the logical level of the written data is held in the memory circuit21. Even if the logical levels supplied to the bit lines 191a and 191bare thereafter changed, the logical level of the held data remainsunchanged since the bit lines 191a and 191b are electrically cut offfrom the memory circuit 21.

A data read operation is now described. The word line 182 is convertedto a high level, so that the transistor 134 forming the read buffercircuit 221 enters a conducting state. Further, the logical level of theinput terminal 205 which is connected to the word line 182 of the NANDgate 15 goes high, whereby the NAND gate 15 outputs a logical level,which is complementary to the value at the terminal 201b of the memorycircuit 21, from its output terminal 203.

When the terminal 201b is at a high logical level, for example, thelogical level at the output terminal 203 of the NAND gate 15 goes high.Thus, the transistor 123 enters a conducting state and the transistor133 enters a cutoff state, whereby the bit line 192 is driven to the VDDpotential, to read high-level data.

When the terminal 201b is at a low logical level, on the other hand, thelogical level at the output terminal 203 of the NAND gate 15 goes high.The transistor 123 enters a cutoff state and the transistor 133 enters aconducting state while the transistor 134 is also in a conducting state,whereby the bit line 192 is driven to the GND potential. Namely,low-level data is read on the bit line 192.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that the read data is driven toa circuit of a next stage. The data read operation is completed in theaforementioned manner.

After the completion of the data read operation, the word line 182 isconverted to a low level, so that the value at the output terminal 203of the NAND gate 15 goes high and the transistor 123 enters a cutoffstate. The transistor 134 also enters a cutoff state, whereby the bitline 192 is electrically disconnected from the read buffer circuit 221.

Thus, it is possible to simultaneously perform data read and writeoperations since the data are read and written from independent ports.The as-stored data are prevented from destruction caused by the readoperation due to the read buffer circuit 171 provided in the memory cellcircuit 221.

The bit line 192 fully swings between high and low levels in response tothe logical level which is supplied to the terminal 201b of the memorycircuit 21. Even if potential difference between the VDD potential andthe GND potential which are supplied by the power supply line 111 andthe grounding conductor 112 is reduced, therefore, it is possible tostably read the data from the bit line 192 with a wide operation margin.

According to this embodiment, it is possible to reduce the number ofwires since no pair of complementary read word lines are requireddissimilarly to the second or third prior art which enables a full swingof the bit line 192.

When the bit line 192 is driven to a high level at the final stage ofthe read buffer circuit 221, a high-speed operation is enabled since thebit line 192 is connected with the power supply line 111 by the singleP-channel MOS transistor 123.

FIG. 4 is a circuit diagram showing a modification of the firstembodiment, in which an operating speed of an N-channel MOS transistoris improved. Referring to FIG. 4, a memory cell circuit 171i is providedwith a read buffer circuit 221i in place of the read buffer circuit 221shown in FIG. 1. This memory cell circuit 171i is connected with a readword line 182i which is supplied with a signal for driving the readbuffer circuit 221i at a low level, in place of the read word line 182.

The read buffer circuit 221i is formed by a NOR gate 15i, an N-channelMOS transistor 123i, and P-channel MOS transistors 133i and 134i.

The transistor 123i has a source which is connected to a groundingconductor 112 to be supplied with a GND potential, while the transistor133i has a source which is connected to a power supply line 111 to besupplied with a VDD potential. The transistor 134i has a source which isconnected with a drain of the transistor 133i, while the transistors123i and 134i have drains which are connected to a bit line 192 incommon. The transistors 123i and 133i have gates which are connected toan output terminal of the NOR gate 15i in common, while the transistor134i has a gate which is connected to a word line 182i. The NOR gate 15ihas an input terminal which is connected with a terminal 201b of amemory circuit 21, and another input terminal which is connected withthe gate of the transistor 134i.

As hereinabove described, the memory cell circuit 171i is formed by thememory circuit 21 and the read buffer circuit 221i. Such a structure isadapted to perform an operation which is complementary to that of thefirst embodiment. When the bit line 192 is driven to a low level, ahigh-speed operation is enabled since the bit line 192 is connected withthe grounding conductor 112 by the single N-channel MOS transistor 123i.

Such a structure is effective when the P-channel MOS transistors havehigh drivability, or a sense amplifier circuit which is connected to thebit line 192 has a sufficiently low threshold value.

(B-2) Second Embodiment

FIG. 5 is a circuit diagram showing the structure of a memory cellcircuit 172 of a multiport memory having a single write port and asingle read port.

Referring to FIG. 5, the memory cell circuit 172 is identical instructure to the memory cell circuit 171 according to the firstembodiment, except that a transistor 133 forming a read buffer circuit222 has a gate which is connected not to an output terminal 203 of aNAND gate 15 but to a terminal 201a of a memory circuit 21.

The operation of the memory cell circuit 172 is now described. A datawrite operation is absolutely identical to that of the first embodiment.

In a data read operation, a word line 182 is converted to a high level,so that a transistor 134 forming the read buffer circuit 222 enters aconducting state. Further, the logical level of an input terminal 205which is connected to a gate of the transistor 134 of the NAND gate 15goes high, so that a logical level which is complementary to thatsupplied to a terminal 201b of the memory circuit 21 is outputted fromthe output terminal 203 of the NAND gate 15.

When the terminal 201b of the memory circuit 21 is supplied with a highlogical level, for example, the logical level at the output terminal 203of the NAND gate 15 goes low. On the other hand, the terminal 201a ofthe memory circuit 21 is supplied with a low logical level, whereby thetransistor 123 enters a conducting state and the transistor 133 enters acutoff state so that the bit line 192 is driven to a VDD potential.Namely, high-level data is read out.

When the terminal 201b is supplied with a low logical level, on theother hand, the logical level at the output terminal 203 of the NANDgate 15 goes high. On the other hand, the terminal 201a is supplied witha high logical level, whereby the transistor 123 enters a cutoff stateand the transistor 133 enters a conducting state while the transistor134 is also in a conducting state, so that the bit line 192 is driven toa GND potential. Namely, low-level data is read out.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that the read data is driven toa circuit of a next stage. The data read operation is completed in theaforementioned manner.

After the completion of the data read operation, the word line 182 isconverted to a low level, so that the value at the output terminal 203of the NAND gate 15 goes high and the transistor 123 enters a cutoffstate. The transistor 134 also enters a cutoff state, so that the bitline 192 is electrically disconnected from the read buffer circuit 222.

The second embodiment having the aforementioned structure performs thesame operation as the first embodiment, whereby an effect identical tothat of the first embodiment can be attained.

FIG. 6 shows a modification of the second embodiment, in which anoperation speed of an N-channel MOS transistor is improved. Thismodification of the second embodiment is identical in structure to thememory cell circuit 221i according to the modification of the firstembodiment shown in FIG. 4, except that a transistor 133i forming a readbuffer circuit 222i has a gate which is connected not to an outputterminal of a NOR gate 15i but to a terminal 201a of a memory circuit21.

When a bit line 192 is driven to a low level, therefore, a high-speedoperation is enabled since the bit line 192 is connected with agrounding conductor 112 by a single N-channel MOS transistor 123i,similarly to the modification of the first embodiment shown in FIG. 4.

(B-3) Third Embodiment

FIG. 7 is a circuit diagram showing the structure of a memory cellcircuit 173 of a multiport memory having a single write port and asingle read port.

The circuit structure of this memory cell circuit 173 is now described.The memory cell circuit 173 comprises a memory circuit 21 and a readbuffer circuit 223.

The memory circuit 21 is formed in a similar manner to that in the firstembodiment. In the read buffer circuit 223, transistors 121, 122, 131and 132 are connected in the manner shown in FIG. 3, to form a NANDgate.

In more concrete terms, gates of the transistors 122 and 132 areconnected in common to form an input terminal 204 of the NAND gate, andthis input terminal 204 is connected with a terminal 201a of the memorycircuit 21. Further, gates of the transistors 121 and 131 are connectedin common to form another input terminal 205 of the NAND gate, and thisinput terminal 205 is connected with a word line 182.

The transistor 123 has a source which is connected to a power supplyline 111 to be supplied with a VDD potential, while the transistor 133has a source which is connected to a drain of the transistor 131. Thetransistors 123 and 133 have drains which are connected to a bit line192 in common. The transistor 123 has a gate which is connected to aterminal 203, which is an output terminal of the NAND gate, while thetransistor 133 has a gate which is connected to the terminal 201a of thememory circuit 21.

In the read buffer circuit 223 having the aforementioned structure, thetransistor 131 also serves as the transistor 134 in the secondembodiment.

The operation of the memory cell circuit 173 is now described. A dataread operation is absolutely identical to that in the first embodiment.In a data read operation, on the other hand, the word line 182 isconverted to a high level, so that the transistor 121 enters a cutoffstate and the transistor 131 enters a conducting state. Thus, a logicallevel which is complementary to that at another terminal 201b of thememory circuit 21 is outputted from the output terminal 203 of the NANDgate.

Namely, when the terminals 201a and 201b of the memory circuit 21 are atlow and high logical levels respectively, the transistor 122 enters acutoff state and the transistor 132 enters a conducting state, so thatthe logical level at the output terminal 203 of the NAND gate goes high.Thus, the transistor 123 enters a conducting state.

On the other hand, the transistor 133 is in a cutoff state, whereby thebit line 192 is driven to the VDD potential to read high-level data.

When the terminals 201a and 201b of the memory circuit 21 are at highand low levels respectively, on the other hand, the transistor 122enters a conducting state and the transistor 132 enters a cutoff state,whereby the logical level at the output terminal 203 of the NAND gategoes high. Thus, the transistor 123 enters a cutoff state.

On the other hand, the transistor 133 is in a conducting state, wherebythe bit line 192 is driven to the GND potential, to read low-level data.

The bit line 192 is connected with a sense amplifier (not shown)directly or through a transfer gate, to drive the read data to a circuitin a next stage. The data read operation is completed in theaforementioned manner.

After the completion of the data read operation, the word line 182 isconverted to a low level, so that the transistor 121 enters a conductingstate and the transistor 131 enters a cutoff state. Namely, the logicallevel at the output terminal 203 of the NAND gate goes high and thetransistor 123 enters a cutoff state. On the other hand, the transistors131 and 123 are in cutoff states, whereby the bit line 192 iselectrically disconnected from the read buffer circuit 223 not only whenthe transistor 133 is in a cutoff state but when the transistor 133 isin a conducting state.

The circuit according to the third embodiment also performs the sameoperation as those of the first and second embodiments, whereby asimilar effect can be attained.

Similarly to the first embodiment, the gate of the transistor 133 may beconnected to the output terminal 203 of the NAND gate, as a matter ofcourse.

It is also possible to obtain a modification which is similar to thoseof the first and second embodiments. The NOR gate 15i in themodification of the second embodiment shown in FIG. 6 may be formed bytransistors, one of which also serves as the transistor 134i.

FIG. 8 is a circuit diagram showing a memory cell circuit 173i accordingto modification of the third embodiment, which is aimed at improvingoperating speeds in N-channel MOS transistors. N-channel MOS transistors121i and 122i and P-channel MOS transistors 131i and 132i form a NORgate. The transistor 131i also serves as the transistor 134i in themodification of the second embodiment shown in FIG. 6.

Similarly to the third embodiment which can attain an effect similar tothat of the second embodiment through a similar operation, themodification of the third embodiment can attain an effect similar tothat of the modification of the second embodiment through a similaroperation.

(B-4) Fourth Embodiment

FIG. 9 is a circuit diagram showing a memory cell circuit 174 of amultiport memory cell having a single write port and two read ports.This memory cell circuit 174, which is similar to the memory cellcircuit 171 according to the first embodiment, is provided with aplurality of read buffer circuits.

The memory cell circuit 174 comprises a memory circuit 21, and readbuffer circuits 221a and 221b. The memory circuit 21 is formed in asimilar manner to that of the first embodiment.

The read buffer circuit 221a is formed by a NAND gate 15a, a P-channelMOS transistor 123a, and N-channel MOS transistors 133a and 134a. Thetransistor 123a has a source which is connected to a power supply line111 to be supplied with a VDD potential. The transistor 134a has asource which is connected to a drain of connected to a drain of thetransistor 133a, which in turn has a source connected to a groundingconductor 112 to be supplied with a GND potential.

The transistors 123a and 134a have drains which are connected to a bitline 192a in common. The transistors 123a and 133a have gates which areconnected to an output terminal 203a of the NAND gate 15a in common. Thetransistor 134a has a gate which is connected to a read word line 182,while the NAND gate 15a has an input terminal 204a which is connectedwith a terminal 201b of the memory circuit 21 and another input terminal205a which is connected with a gate of the transistor 134a.

Similarly, the read buffer circuit 221b is formed by a NAND gate 15b, aP-channel MOS transistor 123b, and N-channel MOS transistors 133b and134b. The transistor 123b has a source which is connected to the powersupply line 111 to be supplied with the VDD potential. The transistor134b has a source which is connected to a drain of the transistor 133b,which in turn has a source connected to the grounding conductor 112 tobe supplied with the GND potential.

The transistors 123b and 134b have drains which are connected to the bitline 192b in common. The transistors 123b and 133b also have gates whichare connected to an output terminal 203b of the NAND gate 15b in common.The transistor 134b has a gate which is connected to a read word line183, while the NAND gate 15b has an input terminal 204b which isconnected with the terminal 201b of the memory circuit 21 and anotherinput terminal 205b which is connected with a gate of the transistor134b.

In operation of the memory cell circuit 174, data are written absolutelyin the same manner as that in the first embodiment. A data readoperation is now briefly described.

The word lines 182 and 183 may simultaneously go high with no problemsince the same correspond to independent ports respectively. The wordline 181 also corresponds to still another independent port, wherebywrite and read operations can be simultaneously performed.

When the read word line 182 is converted to a high level, data held inthe terminal 201b of the memory circuit 21 is outputted on the read bitline 192a. When the read word line 183 is converted to a high level, onthe other hand, data held in the terminal 201b of the memory circuit 21is outputted to the read bit line 192b.

The detail of the read operation is similar to that with the read buffercircuit 221 according to the first embodiment, whereby an effect similarto that of the first embodiment can be attained.

(B-5) Fifth Embodiment

FIG. 10 is a circuit diagram showing a memory cell circuit 175 of amultiport memory having one write port and two read ports. This memorycell circuit 175, which is similar to the memory cell circuit 172according to the second embodiment, is provided with a plurality of readbuffer circuits.

According to the fifth embodiment, gates of transistors 133a and 133b,which are connected to the respective output terminals 203a and 203b ofthe NAND gates 15a and 15b in the fourth embodiment, are connected to aterminal 201a of a memory circuit 21 in common. In other words, therelation of the fifth embodiment to the second embodiment is equivalentto that of the fourth embodiment to the first embodiment.

A data write operation of the memory cell circuit 175 is absolutelysimilar to that of the first embodiment. A data read operation is nowbriefly described.

Similarly to the fourth embodiment, word lines 182 and 183 maysimultaneously go high since the same independently correspond todifferent ports respectively. Still another word line 181 alsocorresponds to still another independent port, whereby data write andread operations can be simultaneously performed.

When the read word line 182 is converted to a high level, data stored inthe terminal 201b of the memory circuit 21 is outputted to a read bitline 192a. When the read word line 183 is converted to a high level, onthe other hand, the data stored in the terminal 201b of the memorycircuit 201a is outputted to another read bit line 192b.

The detail of the data read operation is similar to that of the secondembodiment. Thus, an effect similar to that of the second embodiment canbe attained.

(B-6) Sixth Embodiment

FIG. 11 is a circuit diagram showing a memory cell circuit 176 of amultiport memory having one write port and two read ports. This memorycell circuit 176, which is similar to the memory cell circuit 173according to the third embodiment, is provided with a plurality of readbuffer circuits.

According to the sixth embodiment, portions corresponding to the NANDgates 15a and 15b of the fourth embodiment are formed by transistorsrespectively. In other words, the relation of the sixth embodiment tothe third embodiment is equivalent to that of the fourth embodiment tothe first embodiment.

The circuit structure of the memory cell circuit 176 is now described.This memory cell circuit 176 comprises a memory circuit 21 and readbuffer circuits 223a and 223b.

The memory circuit 21 is formed similarly to the first embodiment. Inthe read buffer circuit 223a, P-channel MOS transistors 121a and 122aand N-channel MOS transistors 131a and 132a are connected in the mannershown in FIG. 3, to form a NAND gate.

The transistors 122a and 132a have gates which are connected to aterminal 201b of the memory circuit 21 in common, while the transistors121a and 131a have gates which are connected to a read word line 182 incommon.

A P-channel MOS transistor 123a has a source which is connected to apower supply line 111 to be supplied with a VDD potential, while anN-channel MOS transistor 133a has a source which is connected to a drainof the transistor 131a. The transistors 123a and 133a have drains whichare connected to a bit line 192a in common. The transistor 123a has agate which is connected to a terminal 203a, serving as an outputterminal of the NAND gate, while the transistor 133a has a gate which isconnected to a terminal 201a of the memory circuit 21. The read buffercircuit 223a is formed in the aforementioned manner.

Similarly, P-channel MOS transistors 121b and 122b and N-channel MOStransistors 131b and 132b are connected in the manner shown in FIG. 3 inthe read buffer circuit 223b, to form a NAND gate.

The transistors 122b and 132b have gates which are connected to theterminal 201b of the memory circuit 21 in common, while the transistors121b and 131b have gates which are connected to a read word line 183 incommon.

A P-channel MOS transistor 123b has a source which is connected to thepower supply line 111 to be supplied with the VDD potential, while anN-channel MOS transistor 133b has a source which is connected to a drainof the transistor 131b. The transistors 123b and 133b have drains whichare connected to a bit line 192b in common. The transistor 123b has agate which is connected to a terminal 203b serving as an output terminalof the NAND gate, while the transistor 133b has a gate which isconnected to the terminal 201a of the memory circuit 21. The read buffercircuit 223b is formed in the aforementioned manner.

In the memory cell circuit 176, a data write operation is absolutelyidentical to that of the first embodiment. A data read operation is nowbriefly described.

When the read word line 182 is converted to a high level, data held inthe terminal 201b of the memory circuit 21 is outputted to the read bitline 192a. When the read word line 183 is converted to a high level, onthe other hand, the data held in the terminal 201b of the memory circuit21 is outputted to the read bit line 192b.

Similarly to the fourth and fifth embodiments, the word lines 182 and183 may simultaneously go high with no problem since the sameindependently correspond to different ports respectively. Since stillanother word line 181 corresponds to still another independent port,further, read and write operations can be simultaneously performed.

The detail of the read operation is similar to that of the thirdembodiment, whereby it is possible to attain an effect similar to thatof the third embodiment.

(B-7) Seventh Embodiment

FIG. 12 is a circuit diagram showing the structure of a memory cellcircuit 177 of a multiport memory having a write port and a read port.

This memory cell circuit 177 comprises memory circuits 21a and 21b, anda read buffer circuit 224.

The memory circuit 21a is formed by a flip-flop circuit includinginverter circuits 14a and 14b whose output terminals 201a and 201b areconnected to input terminals of the counter inverter circuits 14b and14a, to store data.

Similarly, the other memory circuit 21b is formed by a flip-flop circuitincluding inverter circuits 14c and 14d whose output terminals 201c and201d are connected to input terminals of the counter inverter circuits14d and 14c, to store data.

The memory circuits 21a and 21b, which are provided with write accessgates for writing data being received in the write port in the memorycircuits 21a and 21b, are connected with write bit lines 191a and 191bfor transferring write data, and write word lines 181a and 181b forselecting desired ones of the memory circuits 21a and 21b.

The write access gates are formed by N-channel MOS transistors 13a, 13b,13c and 13d. The transistors 13a and 13b have drains which are connectedto the terminals 201a and 201b of the memory circuit 21a respectively,sources which are connected to the write bit lines 191a and 191brespectively, and gates which are connected to the write word line 181ain common.

Similarly, the transistors 13c and 13d have drains which are connectedto the terminals 201c and 201d of the memory circuit 21b respectively,sources which are connected to the bit lines 191a and 191b respectively,and gates which are connected to the word line 181b in common.

The read buffer circuit 224, which is adapted to read the data stored inthe memory circuit 21a and 21b, is connected with a read bit line 192for transferring read data, and read word lines 182a and 182b forselecting desired ones of the memory circuits 21a and 21b in the memorycell circuit 177.

The read buffer circuit 224 is formed by AND gates and a NOR gate whichare represented by an AND-NOR composite circuit 16, a P-channel MOStransistor 123, and N-channel MOS transistors 133, 134 and 139.

The transistor 123 has a source which is connected to a power supplyline 111, to be supplied with a VDD potential. The transistor 133 has asource which is connected to a grounding conductor 112, to be suppliedwith a GND potential.

The transistors 134 and 139 have sources which are connected with adrain of the transistor 133 in common. The transistors 123, 134 and 139have drains which are connected to the bit line 192 in common.

The transistors 123 and 133 have gates which are connected to an outputterminal 206 of the AND-NOR composite circuit 16 in common. Thetransistors 134 and 139 have gates which are connected to the word lines182a and 182b respectively.

The AND-NOR composite circuit 16 has a pair of AND input terminals 209and 210, which are connected with the terminal 201b of the memorycircuit 21a and the gate of the transistor 134 respectively, and anotherpair of AND input terminals 207 and 208 which are connected with thegate of the transistor 139 and the terminal 201d of the memory circuit21b respectively.

FIGS. 13 and 14 illustrate the structure and the operation of theAND-NOR composite circuit 16 shown in FIG. 12. FIG. 13 is a logic symboldiagram of the AND-NOR composite circuit 16, and FIG. 14 is a circuitdiagram showing an exemplary structure of the AND-NOR composite circuit16 formed by MOS transistors.

The circuit structure of the AND-NOR composite circuit 16 is nowdescribed with reference to FIG. 14.

Referring to FIG. 14, P-channel MOS transistors 124 and 125 have sourceswhich are connected to the power supply line 111, to be supplied withthe VDD potential. These transistors 124 and 125 have drains which areconnected in common with sources of P-channel MOS transistors 126 and127. The transistors 126 and 127 have drains which are connected incommon with drains of N-channel MOS transistors 136 and 138, to form theoutput terminal 206.

The transistor 136 has a source which is connected to a drain of thetransistor 135. The transistor 135 has a source which is connected tothe grounding conductor 112, to be supplied with the GND potential. Thetransistor 138 has a source which is connected to a drain of thetransistor 137. The transistor 137 has a source which is connected tothe grounding conductor 112 to be supplied with the GND potential.

Gates of the transistors 124 and 135, 125 and 136, 126 and 137, and 127and 138 are connected with each other in common respectively, to forminput terminals 207, 208, 209 and 210 respectively. The AND-NORcomposite circuit 16 is formed in the aforementioned manner.

The operation of the AND-NOR composite circuit 16 is now described. Whenboth of the input terminals 207 and 208 are supplied with high-leveldata (VDD potential), the transistors 124 and 125 enter cutoff statesand the transistors 135 and 136 enter conducting states. Thus, theoutput terminal 206 is driven to the GND potential regardless of logicallevels which are supplied to the input terminals 209 and 210, to outputlow-level data.

When both of the input terminals 209 and 210 are similarly supplied withhigh-level data (VDD potential), the transistors 126 and 127 entercutoff states and the transistors 137 and 138 enter conducting states.Thus, the output terminal 206 is driven to the GND potential regardlessof values of the input terminals 207 and 208, to output low-level data.

When at least one of the input terminals 207 and 208 is supplied withlow-level data and at least one of the input terminals 209 and 210 isalso supplied with low-level data, at least one of the transistors 124and 125 and at least one of the transistors 126 and 127 enter conductingstates and at least one of the transistors 135 and 136 and at least oneof the transistors 137 and 138 enter cutoff states. Thus, the outputterminal 206 is driven to the VDD potential, to output high-level data.

Namely, the AND-NOR composite 16 so operates that the output terminal206 is supplied with a low logical level when both of the inputterminals 207 and 208 or both of the input terminals 209 and 210 aresupplied with high-level data while high-level data is outputted fromthe output terminal 206 when the terminals are supplied with other data.

Referring again to FIG. 12, the operation of the memory cell circuit 177is now described. A data write operation is similar to that of the fistembodiment. First, the bit lines 191a and 191b are driven to low or highlevels in response to the logical level of write data through a writedriver circuit (not shown) which is connected to the bit lines 191a and191b. At this time, the bit lines 191a and 19b are driven to be in acomplementary relation to each other.

When data is written in the memory circuit 21a, the word line 181a isdriven to a high level. Since no word lines of the same port rise at thesame time, the word line 181b remains at a low level. At this time, theaccess gates 13a and 13b of the memory circuit 21 enter conductingstates, while the access gates 13c and 13d of the memory circuit 21benter cutoff states.

Thus, the terminals 201a and 201b of the memory circuit 21a areelectrically connected with the bit lines 191a and 191b respectively, sothat the data is written in the memory circuit 21a. Similarly, the wordline 181b is converted to a high level and the word line 181a isconverted to a low level when data is written in the memory circuit 21b.The data write operation is completed in the aforementioned manner.

A data read operation is now described. In order to read data stored inthe memory circuit 21a, the word line 182a is converted to a high leveland the transistor 134 forming the read buffer circuit 224 is broughtinto a conducting state. At this time, a logical level which is suppliedto the input terminal 210 connected to the gate of the transistor 134 ofthe AND-NOR composite circuit 16 also goes high.

Since no word lines of the same port rise at the same time, the wordline 182b is driven to a low level. Therefore, the input terminal 207 isat a low logical level, whereby a logical level which is complementaryto that supplied to the terminal 201b of the memory circuit 21a isoutputted from the output terminal 206 of the AND-NOR composite circuit16.

If the terminal 201b of the memory circuit 21 is supplied with a highlogical level, for example, the output terminal 206 of the AND-NORcomposite circuit 16 is supplied with a low logical level. Thus, thetransistor 123 enters a conducting state and the transistor 133 enters acutoff state, whereby the bit line 192 is driven to the VDD potential sothat high-level data is read out.

If the terminal 201b is supplied with a low logical level, on the otherhand, the output terminal 206 of the AND-NOR composite circuit 16 issupplied with a high logical level. Thus, the transistor 123 enters acutoff state and the transistor 133 enters a conducting state while thetransistor 134 is also in a conducting state, whereby the bit line 192is driven to the GND potential so that low-level data is read out.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that the read data is driven toa circuit of a next stage. Data stored in the memory circuit 21a iscompletely read out in the aforementioned operation.

In order to read data stored in the memory circuit 21b, the word line182b is converted to a high level (the word line 182a is converted to alow level).

After completion of the data read operation, both word lines 182a and182b are converted to low levels so that both input terminals 207 and210 are supplied with low logical levels, whereby the value at theoutput terminal 206 of the AND-NOR composite circuit 16 goes high tobring the transistor 123 into a cutoff state. The transistors 134 and139 also enter cutoff states, whereby the bit line 192 is electricallydisconnected from the read buffer circuit 224.

As hereinabove described, the present invention is also applicable to aplurality of write ports, to attain an effect similar to that of thefirst embodiment.

While this embodiment comprises the two memory circuits 21a and 21b, thebit line 192 is pulled up by the single transistor 123. Thus, thecapacity of read bit lines for each memory means can be reduced ascompared with the prior art. Therefore, the operation can be performedat a higher speed, to reduce the access time. Further, charge/dischargecurrents flowing in the read bit lines are also reduced to save thepower.

It is possible to improve an N-channel MOS transistor in speed, in asimilar manner to the modification of the first embodiment.

FIG. 15 is a circuit diagram showing a modification of the seventhembodiment. In a read buffer circuit 224i, an output of an OR-NANDcomposite circuit 16i is supplied to gates of an N-channel MOStransistor 123i and a P-channel MOS transistor 133i in common. P-channelMOS transistors 134i and 139i, which are connected in series to thetransistor 133i, have gates which are connected with read word lines182ia and 182ib respectively.

The read word lines 182ia and 182ib are supplied with signals fordriving the read buffer circuit 224i at a low level, whereby the readbuffer circuit 224i performs an operation which is complementary to thatof the read buffer circuit 224 shown in FIG. 12. Therefore, a memorycell circuit 177i comprising the read buffer circuit 224i and memorycircuits 21a and 21b can also attain an effect similar to that of theseventh embodiment, i.e., similar to that of the first embodiment.

(B-8) Eighth Embodiment

FIG. 16 is a circuit diagram showing the structure of a memory cellcircuit 178 of a multiport memory having a single write port and asingle read port.

The memory cell circuit 178 comprises memory circuits 21a and 21b and aread buffer circuit 225. The structure of this embodiment is identicalto that of the memory cell circuit 177 according to the seventhembodiment shown in FIG. 12, except the structure of the read buffercircuit 225.

The read buffer circuit 225 is formed by AND gates and a NOR gate whichare expressed as an AND-NOR composite circuit 16, P-channel MOStransistors 123 and 128 and N-channel MOS transistors 133, 134, 139 and130.

The transistors 123 and 128 have sources which are connected to a powersupply line 111 in common, to be supplied with a VDD potential. Thetransistors 133 and 130 have sources which are connected to a groundingconductor 112, to be supplied with a GND potential.

The transistors 134 and 139 have sources which are connected with drainsof the transistor 130, 133, respectively. The transistors 123, 128, 134and 139 have drains which are connected to a bit line 192 in common.

The transistors 123, 128, 133 and 130 have gates which are connected toan output terminal 206 of the AND-NOR composite circuit 16 in common.The transistors 134 and 139 have gates which are connected to word lines182a and 182b respectively.

The AND-NOR composite circuit 16 has a pair of AND input terminals 209and 210 which are connected with a terminal 201b of the memory circuit21a and a gate of the transistor 134 respectively. Another pair of ANDinput terminals 207 and 208 are connected with a gate of the transistor139 and a terminal 201d of the memory circuit 21b respectively.

The transistor 123 in the read buffer circuit 224 of the seventhembodiment is replaced by parallel connection of the transistors 123 and128 in the read buffer circuit 225 of the eighth embodiment, while thetransistor 133 in the read buffer circuit 224 of the seventh embodimentis replaced by the transistors 133 and 130 in the read buffer circuit225 of the eighth embodiment.

In other words, it can be said that the read buffer circuit 224 of theseventh embodiment has such a structure that transistors which can alsobe employed in the read buffer circuit 225 of the eighth embodiment areunified. In order to apply the present invention to a memory cellcircuit having a plurality of memory means, either a single or aplurality of P-channel MOS transistors can be employed for pulling upthe read bit line 192. When a plurality of P-channel MOS transistors areemployed for the pullup operation as shown in the eighth embodiment, itis possible to improve ability for driving the bit line 192 to a highlevel.

The operation of the memory cell circuit 178 is now described. A datawrite operation is absolutely similar to that shown in the seventhembodiment.

A data read operation is now described.

In order to read data stored in the memory circuit 21a, the word line182a is converted to a high level so that the transistor 134 forming theread buffer circuit 225 enters a conducting state. On the other hand,the input terminal 210 which is connected to the gate of the transistor134 of the AND-NOR composite circuit 16 also enters a high logical levelstate.

The word line 182b is driven to a low level since word lines of the sameport will not rise at the same time. Therefore, the input terminal 207is supplied with a low logical level, so that a logical level which iscomplementary to that supplied to the terminal 201b of the memorycircuit 21 is outputted from the output terminal 206 of the AND-NORcomposite circuit 16.

When the terminal 201b of the memory circuit 21a is supplied with a highlogical level, for example, the output terminal 206 of the AND-NORcomposite circuit 16 is supplied with a low logical level. Thus, thetransistors 123 and 128 enter conducting states and the transistor 130enters a cutoff state, whereby the bit line 192 is driven to the VDDpotential so that high-level data is read out.

When the terminal 201b is supplied with a low logical level, on theother hand, the output terminal 206 of the AND-NOR composite circuit 16is supplied with a high logical level. Thus, the transistors 123 and 128enter cutoff states and the transistor 130 enters a conducting state.Since the transistor 134 is also in a conducting state, the bit line 192is driven to the GND potential, so that low-level data is read out.

The bit line 192 is connected with a sense amplifier circuit (not shown)directly or through a transfer gate, so that the as-read data is drivento a circuit of a next stage. The data stored in the memory circuit 21ais completely read in the aforementioned manner.

Similarly, the word line 181b is driven to a high level in order to readdata stored in the memory circuit 21b. In more concrete terms, thetransistor 130 is replaced by the transistor 133 in the abovedescription for the case of reading the data from the memory circuit21a.

After completion of the data read operation, both of the word lines 182aand 182b are driven to low levels so that the output terminal 206 of theAND-NOR composite circuit 16 is supplied with a high logical level andthe transistors 123 and 128 enter cutoff states. The transistors 134 and139 also enter cutoff states, whereby the bit line 192 is electricallydisconnected from the read buffer circuit 225.

The operation in the eighth embodiment is substantially similar to thatin the seventh embodiment, whereby an effect similar to that of theseventh embodiment can be attained.

Similarly to the modification of the seventh embodiment, speeds ofN-channel MOS transistors can be improved.

FIG. 17 is a circuit diagram showing a modification of the eightembodiment. In a read buffer circuit 225i, an output of an OR-NANDcomposite circuit 16i is supplied to gates of N-channel MOS transistors123i and 128i and P-channel MOS transistors 133i and 130i in common. AP-channel MOS transistor 134i which is connected in series to thetransistor 130i has a gate which is connected with a read word line182ia. A P-channel MOS transistor 139i which is connected in series tothe transistor 133i has a gate which is connected with a read word line182ib.

The read word lines 182ia and 182ib are supplied with signals fordriving the read buffer circuit 225i at a low level, whereby the readbuffer circuit 225i performs an operation which is complementary to thatof the read buffer circuit 225 shown in FIG. 16. Thus, a memory cellcircuit 178i comprising the read buffer circuit 225i and memory circuits21a and 21b can attain an effect similar to that of the eighthembodiment.

(B-9) Ninth Embodiment

FIG. 18 is a circuit diagram showing a memory cell circuit 179 of amultiport memory having one write port and two read ports. The ninthembodiment has a structure including a plurality of read buffer circuitsprovided in the memory cell circuit 177 shown in the seventh embodiment.In other words, the relation of the ninth embodiment to the seventhembodiment is equivalent to that of the fourth embodiment to the firstembodiment.

The memory cell circuit 179 comprises memory circuits 21a and 21b andread buffer circuits 224a and 224b. The memory circuits 21a and 21b areformed similarly to those in the seventh embodiment. The read buffercircuits 224a and 224b are formed similarly to the read buffer circuit224 shown in the seventh embodiment.

Transistors 134a and 139a of the read buffer circuit 224a have gateswhich are connected to word lines 182a and 182b respectively. A pair ofAND input terminals 209a and 210a of an AND-NOR composite circuit 16aare connected with a terminal 201b of the memory circuit 21a and thegate of the transistor 134a respectively, while another pair of ANDinput terminals 207a and 208a are connected with the gate of thetransistor 139a and a terminal 201d of the memory circuit 21brespectively.

Similarly, transistors 134b and 139b of the read buffer circuit 224bhave gates which are connected to the word lines 183a and 183brespectively. A pair of AND input terminals 209b and 210b of anotherAND-NOR composite circuit 16b are connected with the terminal 201b ofthe memory circuit 21a and the gate of the transistor 134b respectively,while another pair of AND input terminals 207b and 208b are connectedwith the gate of the transistor 139b and the terminal 201d of the memorycircuit 21b respectively.

An operation of the memory cell circuit 179 for writing data isabsolutely identical to that of the seventh embodiment. A case ofreading data is now briefly described.

When at least one of the read word lines 182a and 183a is converted to ahigh level, data held in the terminal 201b of the memory circuit 21a isread out. This data is outputted to a read bit line 192a when the readword line 182a is converted to a high level, while the same is outputtedto another read bit line 192b when the read word line 183a is convertedto a high level.

Similarly, one of the read word lines 182b and 183b is converted to ahigh level in order to read data held in the terminal 201d of the memorycircuit 21b. This data is outputted to the read bit line 192a when theread word line 182b is converted to a high level, while the same isoutputted to the read bit line 192b when the read word line 183b isconverted to a high level.

A detailed read operation is similar to that shown in the seventhembodiment, whereby an effect of the ninth embodiment is obtainedsimilarly to that of the seventh embodiment.

(B-10) Tenth Embodiment

FIG. 19 is a circuit diagram showing a memory cell circuit 170 of amultiport memory having one write port and two read ports. In the tenthembodiment, a plurality of read buffer circuits are provided in acircuit similar to the memory cell circuit 178 according to the eighthembodiment. In other words, the relation of the tenth embodiment to theeighth embodiment is equivalent to that of the fourth embodiment to thefirst embodiment.

The memory cell circuit 170 comprises memory circuits 21a and 21b andread buffer circuits 225a and 225b. The memory circuits 21a and 21b areformed similarly to those in the seventh embodiment. The read buffercircuits 225a and 225b are also formed similarly to the read buffercircuit 225 according to the eighth embodiment.

Transistors 134a and 139a of the read buffer circuit 225a have gateswhich are connected to word lines 182a and 182b respectively. A pair ofAND input terminals 209a and 210a of an AND-NOR composite circuit 16aare connected with a terminal 201b of the memory circuit 21a and thegate of the transistor 134a respectively, while another pair of ANDinput terminals 207a and 208a are connected with the gate of thetransistor 139a and a terminal 201d of the memory circuit 21brespectively.

Similarly to the above, transistors 134b and 139b of the read buffercircuit 225b are connected to the word lines 183a and 183b respectively.A pair of AND input terminals 209b and 210b of another AND-NOR compositecircuit 16b are connected with the terminal 201b of the memory circuit21a and the gate of the transistor 134b respectively, while another pairof AND input terminals 207b and 208b are connected with the gate of thetransistor 139b and the terminal 201d of the memory circuit 21brespectively.

A data write operation of the memory cell circuit 170 is absolutelyidentical to that of the eighth embodiment. A data read operation is nowbriefly described.

When at least either the read word line 182a or 183a is converted to ahigh level, data held in the terminal 201b of the memory circuit 21a isread. The data is outputted to a read bit line 192a when the read wordline 182a is converted to a high level, while the same is outputted to aread bit line 192b when the read word line 183a is converted to a highlevel.

Similarly to the above, either the read word line 182b or 183b isconverted to a high level in order to read data held in the terminal201d of the memory circuit 21b. The data is outputted to the read bitline 192a when the read word line 182b is converted to a high level,while the same is outputted to the read bit line 192b when the read wordline 183b is converted to a high level.

A detailed read operation is similar to that of the eighth embodiment,whereby an effect similar to that of the eighth embodiment can beattained in the tenth embodiment.

C. Description of Occupied Area

The effects attained by the first to tenth embodiments are as describedin the above description. These embodiments are mostly applied to gatearrays. In this case, the effects in operation are attained without muchincreasing occupied areas as compared with the prior art. This is nowdescribed with reference to the drawings.

FIG. 20 is a plan view showing a semiconductor integrated circuit device(chip) 1 forming a CMOS gate array. A number of input/output pads 2 arearranged in the peripheral edge portions of the chip 1, while aplurality of basic cell stages 3 are provided in the center thereof.

FIG. 21 is a plan view showing one of the basic cell stages 3 in anenlarged manner. This figure illustrates a single basic cell stage 3 ina gate array employing a gate isolation system. The single basic cellstage 3 is formed by a plurality of gates 4 and a plurality of P-typediffusion regions 6 which are arranged in a single column, and aplurality of gates 5 and a plurality of N-type diffusion regions 7 whichare arranged in another column.

FIG. 22 is a circuit diagram showing the basic cell stage 3 of FIG. 21.A single gate 4 and P-type diffusion regions 6 provided on both sidesthereof define a single P-channel MOS transistor. On the other hand, asingle gate 5 and N-type diffusion regions 7 provided on both sidesthereof define a single N-channel MOS transistor. In the gate isolationsystem, a potential which is supplied to a gate of a transistor to beisolated is controlled to bring the same into a cutoff state, therebydividing a column of serially connected transistors and forming adesired circuit.

Referring again to FIG. 21, each basic cell BC is formed by a singleN-channel MOS transistor and a single P-channel MOS transistor. Adesired circuit is formed in units of such basic cells BC.

For example, FIG. 23 is a wiring diagram showing two memory cellcircuits 17a, according to the prior art shown in FIG. 32, which areformed by gate arrays. Referring to FIG. 23, thick solid lines showfirst layer wiring and hatched lines show second layer wiringrespectively. Triangles show contact holes connecting gates 4 and 5,P-type diffusion regions 6 and N-type diffusion regions 7 with the firstlayer wires, while quadrangles show through holes connecting the firstlayer wires with the second layer wires. FIGS. 24 to 28 are illustratedin similar manners to the above.

Two memory circuits 17a, each including a single memory circuit 21 (notshown to avoid complicatedness, throughout FIGS. 23 to 28), are formedby 16 basic cells BC. In other words, eight basic cells BC are requiredfor each memory circuit 21.

FIG. 24 is a wiring diagram showing two memory cell circuits 171according to the first embodiment. The two memory cell circuits 171,each including a single memory circuit 21, are formed by 18 basic cellsBC. In other words, nine basic cells BC are required for each memorycircuit 21.

Thus, it is understood that an occupied area of the first embodiment ofthe present invention which is implemented in a gate array is only 9/8times that of the prior art.

FIG. 25 is a wiring diagram showing two memory cell circuits 172according to the second embodiment. The two memory cell circuits 172,each including a single memory circuit 21, are formed by 18 basic cellsBC. Thus, it is understood that an occupied area of the secondembodiment of the present invention which is implemented in a gate arrayis only 9/8 times that of the prior art, similarly to the firstembodiment.

FIG. 26 is a wiring diagram showing two memory cell circuits 173according to the third embodiment. The two memory cell circuits 173,each including a single memory circuit 21, are formed by 18 basic cellsBC. Thus, it is understood that an occupied area of the third embodimentof the present invention which is implemented in a gate array is only9/8 times that of the prior art, similarly to the first and secondembodiments.

FIG. 27 is a wiring diagram showing a memory cell circuit 177 accordingto the seventh embodiment. The memory cell circuit 177, including twomemory means, i.e., memory circuits 21a and 21b, is formed by 18 basiccells BC. In other words, nine basic cells BC are required for singlememory means, similarly to the first to third embodiments.

Thus, it is understood that an occupied area of the seventh embodimentof the present invention which is implemented in a gate array is only9/8 times that of the prior art, similarly to the first to thirdembodiments.

FIG. 28 is a wiring diagram showing a memory cell circuit 178 accordingto the eighth embodiment. The memory cell circuit 178, including twomemory means, i.e., memory circuits 21a and 21b, is formed by 18 basiccells BC. In other words, nine basic cells BC are required for singlememory means, similarly to the seventh embodiment.

Thus, it is understood that an occupied area of the eighth embodiment ofthe present invention which is implemented in a gate array is only 9/8times that of the prior art, similarly to the seventh embodiment.

Thus, the effects in operation of the first to third embodiments and theseventh and eighth embodiments can be attained without much increasingthe occupied areas as compared with the prior art.

D. Description of Embodiments for Improving Transistors of TwoConductivity Types

Description in the item B has been made on embodiments to which thepresent invention is applied noting only either P-channel or N-channelMOS transistors. Description is now made on embodiments noting both ofP-channel and N-channel transistors for improving operational speeds, towhichever logical levels read bit lines are driven.

(D-1) Eleventh Embodiment

FIG. 29 is a circuit diagram showing a memory cell circuit 1701according to an eleventh embodiment of the present invention. In thisembodiment, a read buffer circuit 226 is provided in place of the readbuffer circuit 221 in the structure shown in the first embodiment.

In the read buffer circuit 226, a P-channel MOS transistor 123 and anN-channel MOS transistor 134 are connected in series across a powersupply line 111 and a grounding conductor 112. The transistor 123receives an output of a NAND gate 151 in its gate, while the transistor134 receives an output of a NOR gate 152 in its gate respectively.

The NAND gate 151 has an input terminal 1511 which is connected with aterminal 201b of a memory circuit 21 and another input terminal 1512which is connected with an input terminal of an inverter circuit 153respectively. The NOR gate 152 has an input terminal 1521 which issupplied with an output of the NAND gate 151 and another input terminal1522 which is supplied with an output of the inverter circuit 153respectively. An input terminal of the inverter circuit 153 is connectedwith a read word line 182.

In the memory cell circuit 1701, the memory circuit 21 is formedsimilarly to that of the first embodiment, whereby its read operation issimilar to that of the first embodiment. A read operation of thisembodiment is now described on the basis of Table 1.

                  TABLE 1                                                         ______________________________________                                        Logic of                                                                             Logic of Output  Output                                                                              State of                                                                            State of                                                                            Logic of                            Word   Terminal of Gate of Gate                                                                             TRAN  TRAN  Bit Line                            Line 182                                                                             210b     151     152   123   134   192                                 ______________________________________                                        L      L        H       L     Cut off                                                                             Cut off                                                                             Z                                   L      H        H       L     Cut off                                                                             Cut off                                                                             Z                                   H      L        H       H     Cut off                                                                             Conduct                                                                             L                                   H      H        L       L     Conduct                                                                             Cut off                                                                             H                                   ______________________________________                                    

When the read word line 182 is at a low logical level, output logicallevels of the gates 151 and 152 go high and low respectively withoutdepending on a logical level which is supplied to the terminal 201b ofthe memory circuit 21. In this case, therefore, both of the transistors123 and 134 enter cutoff states and the bit line 192 enters adisconnected state, i.e., a floating state. This is expressed as a "Z"level in Table 1.

When the read word line 182 is at a high logical level, on the otherhand, both outputs of the gates 151 and 152 go to logical levels whichare complementary to that supplied to the terminal 201b of the memorycircuit 21.

When the terminal 201b of the memory circuit 21 is supplied with a lowlogical level, therefore, the transistors 123 and 134 enter cutoff andconducting states respectively, whereby the bit line 192 is suppliedwith a low logical level. When the terminal 201b of the memory circuit21 is supplied with a high logical level, the bit line 192 is suppliedwith a high logical level. Namely, a logical level which is identical tothat supplied to the terminal 201b of the memory circuit 21 is read outfrom the bit line 192.

When a high level is outputted in the bit line 192, only the transistor123 connects the bit line 192 with the power supply line 111. When a lowlevel is outputted in the bit line 192, on the other hand, only thetransistor 134 connects the bit line 192 with the grounding conductor112. Namely, the bit line 192 is driven by a single transistor whicheverlogical level is outputted, whereby the operating speed is improved ascompared with the prior art.

(D-2) Twelfth Embodiment

FIG. 30 is a circuit diagram showing a memory cell circuit 1702according to a twelfth embodiment of the present invention. A readbuffer circuit 227 is provided in place of the read buffer circuit 226in the structure shown in the eleventh embodiment.

The read buffer circuit 227 has such a structure that the NOR gate 152and the inverter circuit 153 in the read buffer circuit 226 are replacedby an AND gate 154. Namely, transistors 123 and 134 are supplied withoutputs of a NAND gate 151 and the AND gate 154 in gates thereofrespectively.

Further, the NAND gate 151 has an input terminal 1511 which is connectedwith a terminal 201b of a memory circuit 21 and another input terminal1512 which is connected with an input terminal 1542 of the AND gate 154respectively. The input terminal 1542 of the AND gate 154 is connectedwith a read word line 182. Another input terminal 1541 of the AND gate154 is supplied with an output of the NAND gate 151.

In the memory cell circuit 1702, the memory circuit 21 is formedsimilarly to that of the first embodiment, whereby its write operationis similar to that of the first embodiment. On the other hand, a readoperation of this embodiment is similar to that of the eleventhembodiment, as shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Logic of                                                                             Logic of Output  Output                                                                              State of                                                                            State of                                                                            Logic of                            Word   Terminal of Gate of Gate                                                                             TRAN  TRAN  Bit Line                            Line 182                                                                             210b     151     152   123   134   192                                 ______________________________________                                        L      L        H       L     Cut off                                                                             Cut off                                                                             Z                                   L      H        H       L     Cut off                                                                             Cut off                                                                             Z                                   H      L        H       H     Cut off                                                                             Conduct                                                                             L                                   H      H        L       L     Conduct                                                                             Cut off                                                                             H                                   ______________________________________                                    

The output of the gate 154 in the twelfth embodiment is absolutelysimilar to that of the gate 152 in the eleventh embodiment. In thetwelfth embodiment of the present invention, therefore, it is possibleto attain an effect identical to that of the eleventh embodiment.

(D-3) Thirteenth Embodiment

FIG. 31 is a circuit diagram showing a memory cell circuit 1703according to a thirteenth embodiment of the present invention. A readbuffer circuit 228 is provided in place of the read buffer circuit 226in the structure shown in the eleventh embodiment.

In this read buffer circuit 228, an input terminal 1541 of an AND gate154 is supplied not with an output of a NAND gate 151, but with alogical level which is supplied to a terminal 201a of a memory circuit21.

In other words, the relation of the thirteenth embodiment to the twelfthembodiment is equivalent to that of the second embodiment to the firstembodiment.

In the memory cell circuit 1703, the memory circuit 21 is formedsimilarly to that of the first embodiment, whereby its write operationis similar to that of the first embodiment. Considering that terminals201a and 201b of the memory circuit 21 are supplied with complementarylogical levels respectively, its read operation is similar to that ofthe twelfth embodiment, as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Logic Logic   Logic                           Logic                           of Word                                                                             of      of      Output                                                                              Output                                                                              State of                                                                            State of                                                                            of Bit                          Line  TRML    TRML    of Gate                                                                             of Gate                                                                             TRAN  TRAN  Line                            182   201b    201a    151   154   123   134   192                             ______________________________________                                        L     L       H       H     L     Cut off                                                                             Cut off                                                                             Z                               L     H       L       H     L     Cut off                                                                             Cut off                                                                             Z                               H     L       H       H     H     Cut off                                                                             Conduct                                                                             L                               H     H       L       L     L     Conduct                                                                             Cut off                                                                             H                               ______________________________________                                    

In the thirteenth embodiment of the present invention, therefore, it ispossible to attain an effect identical to that of the twelfthembodiment, i.e., similar to that of the eleventh embodiment.

E. Other Modifications

While memory cell circuits of two-port structures are expanded to thoseof three-port structures in the fourth to sixth and ninth and tenthembodiments, the port numbers can be further expanded through similarmethods. Also in this case, it is possible to simultaneously carry outread and write operations since all ports are independent of each other.

While each of the aforementioned embodiments has a single write port, itis possible to simultaneously carry out read and write operations alsowhen two or more write ports are provided, since all ports areindependent of each other.

The write access gate may not be formed by an N-channel MOS transistor,but may be formed by a P-channel MOS transistor, by two-channel MOStransistor, or written with a single write bit line.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A memory cell circuit comprising:memory means forstoring memory logic taking either one of first and second logicalvalues being complementary to each other, said memory means having anormal output end for outputting said memory logic; and read meanscoupled to said memory means, said read means including:an outputterminal, a first potential corresponding to said first logical value, asecond potential corresponding to said second logical value, a controlterminal supplying a control signal taking said first logical value forsupplying said output terminal with output logic being identical to saidmemory logic while taking said second logical value for bringing saidoutput terminal into a floating state, a first MOS transistor havingfirst and second current electrodes being connected to said firstpotential and said output terminal respectively, and a controlelectrode, a second MOS transistor having first and second currentelectrodes being connected to said second potential and said outputterminal respectively, and a control electrode, and a logic circuitcoupled to said memory means and said control terminal, for supplyingfirst and second cutoff signals for bringing at least one of said firstand second MOS transistors into a cutoff state to said controlelectrodes of said first and second MOS transistors respectively on thebasis of said control signal and said memory logic, wherein said firstMOS transistor is of a first conductivity type, and said second MOStransistor is of a second conductivity type being complementary to saidfirst conductivity type, and wherein said first and second cutoffsignals are complementary to each other when said control signalperforms control of supplying said output terminal with output logicbeing the same logic as said memory logic, and both said first andsecond cutoff signals are signals being complementary to said memorylogic when said control signal performs control of bringing said outputterminal into said floating state, and wherein said logic circuitcomprises a first logic element receiving said control signal and saidmemory logic and inverting the logical sum thereof for producing saidfirst cutoff signal , wherein said memory means has a reverse output endfor outputting a logical value being complementary to said memory logic,said logic circuit further comprising a second logic element having afirst input being supplied with said control signal and a second inputbeing connected to said reverse output end of said memory means forproducing said second cutoff signal by the logical sum of logical valuesbeing supplied to said first and second inputs thereof.
 2. A memory cellcircuit in accordance with claim 1, further comprising write meansincluding a third MOS transistor having a first current electrode beingconnected to said normal output end, a second current electrode beingsupplied with a predetermined logical value to be written as said memorylogic, and a control electrode being supplied with a write signal fordetermining whether or not said writing is performed.